For existing converters, required output voltage can be obtained by modulating operating frequency of a switch tube in the resonance circuit in a PFM (Pulse Frequency Modulation) mode.
Taking series resonance converter as an example, resonant transformation technique is applied to series resonance DC/DC converter. Since resonant elements are in sinusoidal resonance state, voltage on the switch tube passes zero naturally, and zero voltage turn-on can be realized. At the same time, reverse recovery of output rectifier diodes can be eliminated, and loss of power supply can be reduced and efficiency can be improved. In such topologies, typically, output voltage can be stabilized by modulating operating frequency of a switch tube in a PFM mode. FIG. 1 is the basic form of a full bridge type of series resonance DC/DC converter, wherein the relationship between an output voltage gain M and the operating frequency is:
                    M        =                                            V              o                                      V              in                                =                      0.5                          Qs              ⁢                                                                                    f                                          f                      o                                                        -                                                            f                      o                                        f                                                                                                                          (        a        )                                          f          o                =                  1                      2            ⁢            π            ⁢                                          Lr                ·                Cr                                                                        (        b        )                                Qs        =                              2            ⁢            π            ⁢                                                  ⁢                          f              o                        ⁢                          L              r                        ⁢                          P              o                                            U            o            2                                              (        c        )            
In the above equations (a), (b) and (c), Lr is the value of resonant inductance, and f is the operating frequency of the switch tube, and Cr is the value of resonant capacitance, and Po is an output power.
From equation (a), it can be seen that the higher the operating frequency is, the lower the voltage gain M is when the operating frequency is higher than the resonant frequency. Similarly, the lower the operating frequency is, the lower the voltage gain M is when the operating frequency is lower than the resonant frequency.
As to series resonant converter, there is a problem that it is difficult to stabilize the voltage output in light load or no load state. Output voltage in series resonant topologies decreases as switching frequency increases. When the load of series resonant converter decreases to a light load or no load, the output voltage is stabilized by modulating the operating frequency. Once the operating frequency increases to a certain value, regulating capability of the operating frequency to the output voltage decreases significantly, and the output voltage may even be increased.
Thus, in order to stabilize the output voltage, it is necessary to raise the operating frequency. However, excessively high operating frequency or excessively wide regulating range will make it more difficult to design magnetic elements. Furthermore, the higher the operating frequency is, the greater the circuit loss is. In power supply industry, there are some designs in which dead load is applied on an output terminal in order to stabilize the output voltage in light load or no load state. However, this will increase the no load loss of the power supply and reduce efficiency of the power supply. Also, similar problems exist in half bridge and full bridge series resonance circuits.
In conclusion, simple variable frequency control will result in excessively high operating frequency and excessively wide regulating range, and will make it more difficult to design the magnetic elements, and lower the efficiency of the power supply as well. So, simple variable frequency control can not meet the requirement of a stable output voltage of a converter in light load and no load state.